1. Field of the Invention
The present invention relates to hydromechanical transmissions, and more particularly to an improvement of a hydromechanical transmission of the type wherein low and high speed drive power trains are selectively completed in response to variation of the speed ratio between an input shaft and an output shaft.
2. Description of the Prior Art
In conventional hydromechanical transmissions of this kind, the speed ratio of the output shaft against the input shaft is controlled in accordance with variation of the displacement ratio of an adjustable positive displacment pump-motor and the low speed drive power train is switched over to and from the high speed drive power train when the speed ratio reaches a predetermined value. In the pump-motor, however, it is very difficult to eliminate fluid leakage between a cylinder barrel and a valve plate assembled within the pump-motor. Thus, due to the fluid leakage of the pump-motor, the actual speed ratio conducted under control of the pump-motor differs inevitably from the speed ratio determined in the designing stage. As a result, the switch over between the low and high speed drive power trains may not be surely conducted.